Stable free radicals as activators for photoconductive and photohardenable polymer light-sensitive systems



United States Patent F Int. Cl. G03g /06 U.S. CI. 96-15 17 ClaimsABSTRACT OF THE DISCLOSURE Stable free radicals are used as sensitizersto increase the photoresponsive characteristics of photoconductiveinsulating compositions and photohardenable polymer compositions.

This invention relates to photography and more particularly tophotographic elements comprising novel activators for variousphotosensitive properties of light-sensitive elements.

In the art of photography may compounds have been proposed as activatorsfor increasing the photographic speed and broadening the specialresponse of lightresponsive reactions in various photosensitive systemssuch as (1) layers comprising a resin which is hardened by chemicalcrosslinking in response to light, (2) photoconductive electricallyinsulating layers which undergo change in electrical resistance inresponse to light, (3) photosensitive coatings comprising silver halidesand other metal salts which undergo a change of reducibility byphotographic developers in response to photoexposure.

An object of the present invention is to provide improved photosensitiveelements comprising a stable freeradical as an activator for improvingthe particular photoresponsive characteristics of the photosensitiveelements. This and other objects and advantages will appear from thefollowing detailed description.

In accordance with the present invention, stable free mono-radicals anddi-radicals which are stable under ambient conditions and which can beisolated in pure form, are excellent activators for the photosensitivecharacteristic of various photosensitive systems. T hus, in accord ancewith my invention I provide novel photographic elements comprising anactivating amount of a stable free radical combined in a photosensitivelayer with a photosensitive composition.

Without intending to limit the invention to use of the particulartree-radicals named here, the following are mentioned as preferredexamples selected from the class of stable free-radicals used asactivators in accordance with my invention:

tri-p-tolylaminium perchlorate,

tri-p-tolylaminium picrate,

triphenylaminium perchlorate,

triphenylaminium picrate,

N,N-dibenzyl-4,4-dipyridyl diradical,

tetra-p tolylhydrazinium perchlorate,

di-p-anisylnitrogen oxide,

2,6,3',S-tetra-tbutyl-4-phenoxy-4-methylene-2,S-cyclohe'xadiene-l-oneradical,

2,6,3',5',3",5"-hexa-t-butyl-4',4-diphenoxy-4- methylene-2,5-cyclohexadiene-l-one diradical.

Numerous examples of stable mono-radicals and diradicals have beendescribed in the literature, for example: E. Weitz, Angew. Chem, 66,658-677 (1954); R. Kuhn, ibid., 66, 678-679 (1954), and E. S. Gould,Mechanism and structure in Organic Chemistry, Henry Holt Co., New York(1959), Chapter 16.

3 434",833| Patented Mar. 25, 1969 In order for the activator to beeffective, it must be in its free radical state at the time and underthe conditions in which the photoexposure occurs. Accordingly, the freeradicals useful in my invention are stable in the free-radical stateunder ordinary atmospheric conditions of temperature and pressure andare stable in the environment of the composition of the photosensitivelayer in which they are incorporated. By the term stable free radical wemeans that the radical is capable of remaining essentially in its freeradical state, under ordinary circumstances, in the photosensitive layerin which it is intended for use, at least until photoexposure.

The amount of stable free radical needed in the photosensitive layer toproduce an activating effect will usually be relatively small withrespect to the amount of sensitive material. While the sensitizingamount will vary depending on the particular system in which theactivator is used, generally the optimum amount for a givenlight-sensitive system will be in the range from about 0.1 to about 10percent by weight of the photosensitive component present in the coatedlayer with the activator.

Among the advantages obtained by activation of photosensitive systems bystable free radicals is that relatively higher photographic speeds areobtained by means of these materials in a variety of photosensitivesystems.

Following are specific examples describing some preferred embodiments ofthe present invention.

EXAMPLE I A photohardenable polymer which undergoes crosslinking inresponse to incident light, consisting of poly (vinyl alcohol) in whichhydroxyl groups are substituted to the extent of 12 percent withacetate, 50 percent with azidobenzoate, and 32 percent with dimethylglutarate- 2- urethane was mixed in a 2 percent solution withmethylethyl ketone and activated by the addition of 0.2 percent oftri-p-tolylaminium perchlorate. The resulting dope was coated on a filmbase and the sensitivity of the film to light was determined. The glassfactor of the film described above was 50 and the film responded toactinic wavelengths in the range from 270-400 mu. An unsensitizedcontrol sample had a glass factor of 25 and the actinic wavelengths towhich this sample was sensitive ranged from 280330 mg.

EXAMPLE II A photoconductive system was prepared as follows:

A coating solution was made with 0.5 gram of triphenylamine (an organicphotoconductor) and 2.0 grams of polystyrene dissolved in 15 ml. oftetrahydrofuran with the addition of 0.025 gram of tri-p-tolylaminiumperchlorate. The solution was coated under red safelights onto aluminumfoil. After drying, the coating was stored in the dark. The coating wascharged under a negative corona and then exposed to light through aphotographic negative and then was toned by conventional magnetic brushdevelopment. A good image was obtained using exposure time of one secondto a tungsten source of 30-ft. candle intensity. With a control samplecontaining none of the free-radical activator and tested by the sameprocedure, no image was obtained.

EXAMPLE III A photoconductive element was prepared and tested as inExample II except that triphenylaminium perchlorate was substituted asan activator instead of tri-p-tolylaminium perchlorate. Also, sampleswere prepared substituting Company) for polystyrene. All of the variousresin and activator combinations produced acceptable results using onlyslightly different exposure times for the various samples.

EXAMPLE IV Another photoconducting element was prepared as described inExample 11 except 2,6,3,5'-tetra-t-butyl-4'- phenoxy 4 methylene 2,5cyclohexadiene 1 one radical was substituted for tri-ptolylaminiumperchlorate. The test results were essentially the same as thosedescribed in Examples II and III above.

The examples above demonstrate the versatility of free radicals asactivators for various types of photoresponsive systems in which therespective photosensitive responses may be entirely different. Comparefor example the photochemical response in Example I and thephotoelectric responses in Examples II, III, and IV. My invention is notintended to be restricted to use of free radicals in any particularphotosensitive system but is applicable generally to improvephotoresponse in photographic elements of various types.

'Stable free radical activators of the invention may be used inphotohardenable layers of various kinds such as those used for makinglithographic plates, for example: photosensitive crosslinking polymersystems and photopolymerizable monomer systems. The activators may beused to improve photoresponse in various photoconductiveelectrophotographic layers such as zinc oxide in resin systems,organic-photoconductor-in-resin systems, polymeric photoconductorsystems, inorganic photoconductive coatings and the like. They may beemployed to improve photoresponse in a variety of silver halidephotographic systems, and the like. Generally, the activators are usefulto improve photoresponse in many difierent photosensitive systems.

It will be understood that modifications and variations may be madewithin the scope of the invention as described above and as defined inthe following claims.

I claim:

1. A photographic element comprising a support and coated thereon alayer comprising in combination aphotosensitive composition selectedfrom the group consisting of photoconductive insulating compositions andphotohardenable polymer compositions and an activating amount of astable free radical as a sensitizer for the photoresponsive property ofsaid photosensitive composition.

2. A photographic element comprising a support and coated thereon aphotoconductive insulating layer having dispersed therein a stable freeradical as an activator for the photoconductive property of said layer.

3. The photographic element defined in claim 1 where in said stable freeradical is tri-ptolylaminium perchlorate.

4. The photographic element defined in claim 1 Wherein said stable freeradical is tri-p-tolylaminium picrate.

5. The photographic element defined in claim 1 wherein said stable freeradical is triphenylaminium perchlorate.

6. The photographic element defined in claim 1 wherein said stable freeradical is triphenylaminium picrate.

7. The photographic element defined in claim 1 wherein said stable freeradical is N,N'-dibenzyl-4,4-dipyridyl diradical.

8. The photographic element defined in claim 1 wherein said stable freeradical is tetra-p-tolylhydrazinium perchlorate.

9. The photographic element defined in claim 1 wherein said stable freeradical is dip-anisylnitrogen oxide.

10. The photographic element defined in claim 1 wherein said stable freeradical is 2,6,3,5-tetra-t-butyl-4-phenoxy 4 methylene 2,5cyclohexadiene 1 one radical.

11. The photographic element defined in claim 1 Wherein said stable freeradical is 2,6,3',5',3",5"-hexa-t-butyl- 4,4 diphenoxy 4 methylene 2,5cyclohexadienel-one diradical.

12. A photoconductive element as defined in claim 3 wherein saidphotoconductive insulating layer comprises triphenylamine dispersed inan insulating resin binder and said free radical is tri-p-tolylaminiumperchlorate.

13. A photoconductive element as defined in claim 3 wherein saidphotoconductive insulating layer comprises triphenylamine dispersed inan insulating resin binder and said free radical is triphenylaminiumperchlorate.

14. A photoconductive element as defined in claim 3 wherein saidphotoconductive insulating layer comprises triphenylamine dispersed inan insulating resin binder and said free radical is2,6,3',5-tetra-t-butyl-4,4"-diphenoxy-4-methylene-2,5-cyclohexadiene-l-one diradical.

15. A photographic element having a photohardenable layer comprising aphotocrosslinkable polymer and dispersed therein a stable free radicalin a sensitizing amount.

16. A photographic element as defined in claim 15 wherein said polymeris a poly (vinyl alcohol) having substituted azidobenzoate substitutedat some hydroxyl positions and said free radical is tri-p-tol-ylaminiumperchlorate.

17. A photoconductive element as defined in claim 3 wherein thephotoconductive insulating layer comprises an organic photoconductordispersed in a binder and the stable free radical is selected from thegroup consisting of:

tri-p-tolylaminiurn perchlorate,

tri-p-tolylaminium picrate,

triphenylaminium perchlorate,

triphenylaminium picrate,

N,N-dibenzyl-4,4-dipyridyl diradical,

tetra-p-tolylhydrazinium perchlorate,

di-p-anisylnitrogen oxide,

2,6,3,5 tetra t butyl 4' phenoxy 4 methylene- 2,5-cyclohexadiene-l-oneradical and 2,6,3,5,3",5" hexa t butyl 4,4" diphenoxy 4-meth-ylene-Z,5-cyclohexadiene-l-one diradical.

References Cited UNITED STATES PATENTS 4/1965 Klupfel 'et a1. 961 2/1966Fox et al. 260576 U.S. Cl. X.R.

